1. Field of the Invention
This invention relates to methods and apparatus for charging moving filaments.
2. Description of the Prior Art
It is known to make nonwoven fabrics from staple or continuous filaments by passing the filaments through air nozzles to drive the filaments onto a foraminous belt where a nonwoven web is formed as the belt moves past the air nozzles. The web formed on the belt is subsequently bonded in a conventional manner to increase the strength and enhance other properties of the nonwoven web. In order to obtain nonwoven webs of highest quality the filaments being applied to the foraminous belt should be separated from each other prior to contact with the belt. It is known that separation of the filaments can be achieved by use of triboelectricity or by the use of a corona discharge system wherein a high voltage is used to establish an electric field through which the filaments pass.
U.S. Pat. No. 3,163,753, among other patents, discloses a process for charging filaments being fed to a web laydown zone wherein the filaments are pulled under tension in a wide single-filament layer across a target electrode through an electric field which is established by a plurality of corona discharge electrodes spaced laterally across the layer, i.e., the corona discharge electrodes are positioned in a row extending perpendicular to the direction of travel of the filaments. This electrode arrangement is necessary to charge all of the filaments in the layer.
U.S. Pat. No. 3,689,608 is typical of a number of patents which show apparatus wherein plexifilaments from a spinnerette are projected onto a deflector which opens the plexifilaments into a wide configuration. The spread plexifilament then falls past a target electrode where an electric charge is applied. To charge the spread plexifilament, an array of discharge electrodes positioned in a circle concentric with the deflector is used. Since the plexifilament seems to spread more or less radially from the deflector, this arrangement amounts to a lateral positioning of the electrodes across the path of travel of the plexifilament, such as in U.S. Pat. No. 3,163,753.
One of the major problems encountered in making nonwoven fabrics of the type described is the problem of fabric uniformity. For example, filaments which stick together or are laid into the nonwoven fabric too close to other filaments can easily give the fabric a ropy appearance which will make the fabric unsalable. In the present invention a very high charge is imparted to each of the filaments to insure good filament separation.